Switch droiddoc to use SdkHeaderJars()

Previously, droiddoc was using SdkImplementationJars() to get a JAR to
add to the classpath in order to resolve unknown references in the
source.

Given that SdkHeaderJars() returns jars created by Turbine which are
(for compilation at least) functionally identical to the implementation
jars there is no point in using the implementation jars.

Bug: 148080325
Test: m nothing
Change-Id: I2b718cc1445c04e849dcb0b1f53bba2b0bd90c95